Patent GB-104571-A (GB)
Title: Protective Devices for Alternating Current Electric Systems.
Inventor: MCCOLL ALBERT EDWARD
IPC: H02H3/28
Filed: 1916-03-31
Abstract: 104,571. McColl, A. E. March 31, 1916. Protective arrangements.--In the protection of transformers, generators, and feeders for singlephase or polyphase systems, a biassed or restrained reverse-current relay R<1>, Fig. 1, is employed of which the elements E<1>, E<2> are energized from the secondaries of series transformers T', T<2> having different characteristics and connected in the two sides or ends of the protected apparatus or circuit, so that on fault development, the fault current must have a value which is a predetermined percentage of the load current before the restraint is overcome and the relay becomes operative, whereas if the apparatus or circuit remains healthy and abnormal currents flow therein, the restraint is still further increased and additional stability given to the protective means by reason of the difference in the characteristics of the transformers. In the arrangement shown in Fig. 1 for the protection of a transformer A, B, the current transformer T' in the primary side has 5 per cent. less capacity than the transformer T<2> in the secondary side, and the transformers are connected to the relay as shown so that the 5 per cent. excess current from the transformer T<2> flows through the element E' in a direction to restrain the relay, while the element E<2> is held at relatively constant polarity by the current from the transformer T'. Should a fault develop when the transformer is supplying load alone, it is necessary for the fault current to exceed 5 per cent. of the load before the transformer T<1>, sending its excess current through the element E' in the reverse direction, causes the relay to operate and open switches S<1>, S<2>. On the flow of excessive currents, the transformer T<1> fails to maintain its ratio, whereas the transformer T<2> does maintain its ratio, the transformers being designed with these different characteristics. Consequently, with very large currents contributed by the healthy apparatus to a fault beyond its terminals, the relatively increased current supplied to the relay by the transformer T<2> increases the percentage restraint of the relay. A fuse F' affords time-element overload protection on sustained overload by diverting the current from the transformer T<1> through the element E<1>. In the case of a star-connected generator, the transformers corresponding to T' are placed in each phase at the star-point end of the generator. The application to a single feeder is shown in Fig. 3, in which the biassed reverse-current relay is placed at the supply end Y of the feeder and serves to open switches at both ends, that at the far end being actuated by means of a pilot wire. Arrangements are also described in which, instead of obtaining the restraint by an unbalanced circulating- current system, the current transformer circuits are separated and the bias of the relay afforded by providing the relay with a third or restraining element. The Provisional Specification describes also systems for the protection of parallel feeders by means of mechanical balance or by wattmetertype relays. Specification 104,518 is referred to.